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EFFECTS OF PULSED LASER IRRADIATION ON THERMAL OXIDES OF SILICON D. LLOYD CROSTHWAIT, RAJIV R. SHAH, GEORGE A. BROWN AND ROBERT MAYS, JR. Semiconductor Research and Development Laboratories, Texas Instruments Incorporated, P.O. Box 225012, Dallas, Texas, 75265

ABSTRACT The electrical effects associated with pulsed laser irradiation of thermal oxides of silicon were investigated. Three of the functions performed by thermal oxides are control of field effects, junction passivation and electrical isolation. Representative oxide thicknesses suitable for these applications were studied: - 100 nm; - 500 nm and >, 1.0 pm respectively. The oxides were irradiated by a repetitively Q-switched Nd 3+:YAG laser operated at the fundamental frequency. The key laser parameters of energy and power density were varied over a wide range, from no change to the onset of discernible change and irreversible damage. The range of energy densities over which the oxide was not electrically degraded was established for each thickness of the oxide. This range of energy densities is compared to the useful range of energy densities for annealing polysilicon and for activating an implant through an oxide. The electrical data was obtained using a mercury probe and through the analysis of a large number of MOS capacitors formed on the irradiated oxides. Parameters measured include: breakdown voltages, surface state charge densities and flat band voltages. INTRODUCTION Numerous potential advantages of laser processing have been identified for semiconductor device structures and processes. The successful exploitation of these advantages is contingent on the compatibility of laser processing with conventional process steps which are required to fabricate the complete device structures which comprise the individual elements of an integrated circuit. The elements of integrated circuits are patterned to minimum dimensions in the 3-10-pm range. The spot sizes of both pulsed and CW lasers which are used for laser processing are nominally in the 50-200 pm diameter range. Thus, except for the case of annealing unpatterned films, not only the effect on the layer of interest must be considered but also the effects on underlying regions of the device structure which are exposed as a consequence of patterning must be understood. The purpose of this paper is to investigate the effects of pulsed laser irradiation on the thermal oxides of silicon and on the silicon oxide/silicon interface to undamaged, uniformly doped single crystal silicon. Other workers have reported on various aspects of oxide effects. These include: thermal oxide over implanted silicon,[ II low temperature deposited oxides over implanted silicon [2] and thermal oxide over diffused silicon.[31 In all cases, other investigators have primarily reported on the material changes in a power regime in which visual changes were readily apparent. This work is primarily concerned with electrical changes in the structures which precede the onset of visual change. For the purpose of discussing the sal